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HOCON - my parser and rendering contributions

05 Oct 2026 . category: dev . Comments
#scala #hocon #config #opensource

TLDR: As of 8 October 2026 I have fixed the renderer in sconfig (Scala port of HOCON): 43 PRs, 23 merged. The port and the Java original lightbend/config share their bugs, so I probe both and send the fix to both: 17 PRs upstream, 3 merged. Bugfixes get merged there, features don’t. AI agents did much of the typing; every claim was checked by running code.

Open-source PRs by kastoestoramadus, live from GitHub

This follows HOCON - the config format YAML should have been.

Same bug in both

The Scala port sconfig exists because Scala.js and Scala Native have no JVM and cannot run the Java library, and because Lightbend keeps lightbend/config in maintenance only: its README calls the library “feature complete”, promises to keep it running on new JVM versions and to “rarely make any other changes”. The Scala code is a line-by-line port, so a bug found in one is almost always in the other.

My process: write a probe (same config, same render options), run it against both. Identical misbehaviour means the bug is upstream, and I open twin PRs. In the other direction, a fix merged upstream has to be ported back (about 16 of my sconfig PRs, 10 of them literally titled “Port …”). Once the loop closed fully: I reported #829, the maintainer fixed it in #841, and I ported it as sconfig#590.

Twin pairs open now:

sconfig lightbend/config The bug, in one line
#599 #869 partially resolved merges render as text that cannot be re-parsed
#600 #870 merge entries ignore render options ("a" : 1 next to sib=0)
#601 #871 merged a comment above += is rendered twice
#634 #872 bare Infinity/NaN make the JSON output invalid
#635 #873 getLong silently clamps out-of-range values
#636 #874 deep nesting ends in StackOverflowError

Bug example

ConfigRenderOptions.concise is documented to give valid JSON, but an overflowing double renders as a token no JSON parser accepts (com.typesafe:config 1.4.9):

ConfigFactory.parseString("a = 1e999").root().render(ConfigRenderOptions.concise())

{"a":Infinity}          // before
{"a":"Infinity"}        // after sconfig#634 / lightbend#872

What gets merged upstream

Three of my PRs were merged into lightbend/config, all of them ports of sconfig fixes, all bugfixes, each with the full sbt test doc run in the description (593 to 603 tests). #867 and #871 add a test that fails without the fix; #866 only reduces allocations, so the existing tests cover it:

lightbend/config Fix Size Open to merged Ported from
#867 list comments get an extra space on every render round trip 2 files 46 minutes sconfig#472
#871 a comment above a += 2 is rendered twice 4 files 3 days sconfig#601
#866 getDuration and getMemorySize compile a regex on every read 1 file 3 days sconfig#473

My feature PR #815 (formatting) took five months to get a reply, which called it less intrusive than expected and asked whether I would take it to a full implementation. Three months later: “we at Akka do not have capacity to take this forward”. I closed it myself on 6 October, once #841 had fixed the problem it worked around.

What the reviews asked for

From the two reviews that asked for changes (#871, #866):

  • Put the tests into the existing test classes (ConcatenationTest, ConfParserTest, ConfigTest), not into a new single-purpose one.
  • Cover the edge cases the tests missed: a trailing same-line comment (a += 2 # c) and a dotted path (x.a += 2).
  • Follow the code’s conventions: UPPER_SNAKE_CASE for static constants, the helpers the neighbouring tests use.
  • Trim the PR description to the problem and the fix.

The formatter that started all this is coming along in hocon-fmt ;)

Me

Waldemar Wosiński - Scala & Big Data engineer, father and motorcyclist ;)